1. Field of the Invention
The present invention relates to a circuit breaker primarily housed in electrical equipment such as a battery pack to cut-off current flow when temperature becomes greater than a preset value.
2. Description of the Related Art
The safety level of battery packs and apparatus such as electric motors can be improved by cutting-off current flow in abnormally high-temperature conditions. This can be implemented by using a circuit breaker that disconnects (switches OFF) contact connection at a set temperature. For example, since abnormal charging and discharging of a battery pack housing lithium ion batteries can result in battery heating, a circuit breaker is included as a protection device to cut-off current flow at high-temperature allowing the battery pack to be used safely. In addition, since temperature can become abnormally high when a device such as a motor is overloaded and/or passes excessive current, that current can be cut-off via a circuit breaker to protect the motor and allow it to be used safely.
Targeting these types of applications, a circuit breaker has been developed that detects temperature rise with a bimetallic strip and separates a moving contact from a stationary contact to break connection and switch the circuit breaker OFF (JP 2002-56755 A).
The cited circuit breaker is shown in the cross-section views of FIGS. 1 and 2. FIG. 1 shows the bimetallic strip 108 in the concave downward (under-cambered) state, where the moving contact 107 is connected to the stationary contact 105 to put the circuit breaker in the ON-state. FIG. 2 shows curvature reversal of the bimetallic strip 108, which becomes concave upward to disconnect the moving contact 107 from the stationary contact 105 and put the circuit breaker in the OFF-state.
See also WO2014/171515A1.
When a set temperature is exceeded in the circuit breaker shown in FIGS. 1 and 2, curvature of the bimetallic strip 108 reverses (becomes concave upward) to push the moving contact metal plate 106 upward and separate the moving contact 107 from the stationary contact 105 putting the circuit breaker in the OFF state to cut-off current flow. When temperature is reduced and the bimetallic strip 108 returns to its original (concave downward) shape, spring-behavior of the resiliently bent moving contact metal plate 106 returns the moving contact 107 to the ON-state in contact with the stationary contact 105. When the moving contact metal plate 106 is not pushed upward by the bimetallic strip 108, namely when the curvature of the concave downward bimetallic strip 108 is not reversed by high-temperature, the moving contact metal plate 106 resiliently presses the moving contact 107 against the stationary contact 105. Specifically, under these conditions, the moving contact 107 is held in contact with the stationary contact 105 by the spring-loaded resilient characteristics of the moving contact metal plate 106 keeping the circuit breaker in the ON-state.
As described above, a circuit breaker, which switches to the OFF state when bimetallic strip curvature reverses and has the moving contact metal plate pressing resiliently against the stationary contact in the ON-state, switches according to the characteristics shown in FIG. 3. Specifically, the circuit breaker shown in FIG. 3 switches from the ON-state to the OFF state to cut-off current flow in the unsafe condition where temperature rises to the OFF-temperature (Ta), and resets to the ON-state allowing safe operation to be resumed when temperature drops to the ON-temperature (Tb). Accordingly, this circuit breaker is ideally suited for use in apparatus such as a battery pack. Note that a fuse can also be used as a protection device in the same manner as a circuit breaker to cut-off current flow under high-temperature conditions. However, a fuse cannot reset to the ON-state once it has cut-off current flow (blown) at a high-temperature. Consequently, it has the drawback that use as a protection device in a battery pack does not allow operation to be resumed after high-temperature excursion even when temperature drops to levels allowing safe battery pack operation.
In a circuit breaker that resets to the ON-state allowing operation to be resumed when temperature drops after current cut-off at a high-temperature, it is important to keep the ON-temperature (Tb), where the circuit breaker resets to the ON-state, within a given range. However, as shown by the broken-lines in FIG. 3, when a related-art circuit breaker is exposed to high-temperature, elasticity degradation in the moving contact metal plate results in a lower ON-temperature (Tb′) and a larger temperature difference (Ta−Tb′) between the OFF-temperature (Ta) for current cut-off and the ON-temperature (Tb′) for reset. Specifically, this related art circuit breaker has the drawbacks that hysteresis increases in the switching temperature characteristics, a consistent ON-temperature (Tb′) cannot be maintained, and ON-temperature (Tb′) variation increases. Non-uniform degradation in moving contact metal plate elasticity and inconsistent ON-temperature reduction are due to uneven reduction in the spring back force resiliently pressing the bimetallic strip towards the reset (ON) position. When temperature drops after high-temperature excursion, the bimetallic strip recovers from reversed (concave upward) curvature to its normal (concave downward) shape. During that recovery, the moving contact metal plate resiliently applies pressure (spring back force) on the bimetallic strip promoting its return to the normal (concave downward) state. However, when the resilient spring back force of the moving contact metal plate decreases non-uniformly, the bimetallic strip cannot quickly recover to the ON-state at a given temperature and ON-temperature decreases in an inconsistent manner. Detrimental effects can result from inconsistent reduction in the reset ON-temperature. For example, when temperature drops after high-temperature excursion in a battery, the circuit breaker can maintain current cut-off preventing battery operation even after temperature has dropped to a value allowing battery use.
During the assembly of equipment using a circuit breaker, the circuit breaker is exposed to a thermal environment during process steps such as solder reflow. A related-art circuit breaker with a phosphor bronze moving contact metal plate has the drawback that after thermal exposure such as solder reflow, the switching temperature for reset to the ON-state from the OFF-state, namely the ON-temperature (Tb′), is reduced in an inconsistent manner and the temperature difference (Ta−Tb′) from the ON-temperature (Ta) is increased (also in an inconsistent manner). A circuit breaker with inconsistently reduced ON-temperature (Tb′) and increased hysterises may not be able to reset to the ON-state after switching to the OFF-state at high-temperature even though temperature has dropped to values allowing safe operation. This circuit breaker cannot be used conveniently in various thermal environments. The ON-temperature of a circuit breaker that has switched to the OFF-state can be increased by increasing the OFF-temperature for current cut-off. However, if the OFF-temperature is increased in a circuit breaker with large hysterises, the circuit breaker provided as a protection device, for example in a battery application, has the negative feature that it is unable to ensure battery safety at high-temperature. This is because the circuit breaker is unable to cut-off current even when battery temperature increases to a high value where current should be cut-off.
Incidentally, to reduce variation in the bimetallic strip activation temperature for circuit breaker switching, a manufacturing method that includes bimetallic strip heat-treatment during assembly has been developed (See WO2014/171515A1).
The circuit breaker cited in this patent publication uses a bimetallic strip heat-treated at a temperature 30° C. to 100° C. higher than the temperature of the solder reflow oven. This can reduce shift in bimetallic strip activation temperature for circuit breaker contact switching even after high-temperature exposure such as after circuit board solder-attachment in the reflow oven. This is because once the bimetallic strip has been heat-treated; the temperature for curvature reversal does not change even when exposed to a high-temperature environment. In this circuit breaker, the activation temperature for bimetallic strip curvature reversal to switch the moving contact to the OFF-state does not shift.
However, the circuit breaker described above cannot maintain a constant temperature for returning the bimetallic strip to normal (concave downward) curvature and switching the contacts to the ON-state, and also cannot keep the temperature for switching (resetting) to the ON-state within a given range. This is because the temperature for resetting the OFF-state circuit breaker back to the ON-state depends not only on bimetallic strip characteristics, but also on interaction between the bimetallic strip and restoring pressure applied by the moving contact metal plate. To reduce and stabilize contact resistance (R) in the ON-state, the moving contact resiliently applies pressure on the stationary contact. For example, for a small circuit breaker installed in a battery pack, contact resistance (R) can be restrained to several mΩ by application of 20 g to 30 g of pressure with the moving contact on the stationary contact. However, if moving contact pressure drops to half that value, contact resistance (R) will increase drastically on the order of tens of mΩ. For this reason, conditions are established to maintain resilient pressure on the stationary contact with the moving contact metal plate. Under these conditions, after bimetallic strip curvature reversal and switching to the OFF-state, the bimetallic strip is resiliently pressed toward the (downward curvature) reset direction by the moving contact metal plate. Consequently, if the elastic restoring force of the moving contact metal plate decreases and less pressure is applied (in the reset direction) on the curvature reversed bimetallic strip, temperature for returning the bimetallic strip to its normal (downward curvature) is lowered. Therefore, while a circuit breaker housing a heat-treated bimetallic strip can maintain a constant activation temperature to reverse curvature and switch the contacts to the OFF-state in a high-temperature condition, it has the drawback that it cannot maintain the reset temperature within a given temperature range to switch the contacts back to the ON-state after the bimetallic strip has reversed curvature and switched the contacts to the OFF-state.
It is one object of the present invention to provide a circuit breaker fabrication method and method of manufacturing a battery pack housing that circuit breaker that can minimize shift in the temperature for reset to the ON-state after switching to the OFF-state in a high-temperature environment. Another important object of the present invention to provide a circuit breaker fabrication method and method of manufacturing a battery pack housing that circuit breaker that can prevent high-temperature induced hysterises increase due to ON-temperature reduction, and can reliably cut-off current when protection device temperature rises abnormally while rapidly resetting to the ON-state when temperature drops to values allowing operation.